Light-coloured to white wooden material panels

ABSTRACT

Light to white wood-base boards which are produced from bleached wood fibers and/or beater dyed with a white pigment.

The present invention relates to light to white wood-base boards which have been produced from bleached wood fibers and/or have been beater-dyed with a white pigment.

In the area of wood-base materials, the market in so-called medium density fiberboards (MDF board) and high density fiberboards (HDF board) is greatly expanding. The quantities produced have more than tripled in the last ten years.

MDF and HDF boards can be processed like conventional particleboards. Owing to their uniform structure, however, they are also suitable for the production of profiled parts and are therefore increasingly becoming established in furniture construction. Thus, for example, articles for furnishing rooms and for decorative purposes (for example in trade fair construction), but also higher-quality furniture, are produced from these boards and then, in order to keep the wood-like structure visible, only coated with a colorless finish or coated with an overlay.

Of course, depending on the type of wood used, these boards have a more or less pronounced brown color, which is only of little esthetic value in the furniture sector.

By beater dyeing with the pigment disclosed in WO-A-04/35276 and dye-comprising colorant compositions, the natural color of the wood fibers can be compensated. In this way, it is possible to obtain colored, completely dyed-through, lightfast and hence esthetically high-quality MDF boards which are suitable for the production of articles having a long life, for example of pieces of furniture for the living area.

However, light or even white wood-base boards, in particular MDF boards, were not available to date. These boards would, however, be of particular interest especially for the production of pieces of furniture and interior decoration objects, for example for kitchen or bathroom.

Although the production of white paper by bleaching of the paper pulp and addition of white pigments or white fillers, blue colorants and optical brighteners has long been known and is described, for example, in Paper Trade Journal, 145, pages 26-27 (1961), those skilled in the art did not expect white wood-base boards, in particular MDF boards, to be obtainable in this manner.

Thus, the wood fibers used in the production of MDF boards still comprise substances (e.g. lignin) which are sensitive to oxidation and hence to discoloration and which are no longer comprised in the paper pulp. Moreover, MDF boards are produced at far higher temperatures (about 200° C.) than paper, with the result that, in addition to the natural brown color of the wood fibers, a process-related brown coloration which is due to the oxidation and thermal decomposition of wood ingredients occurs. Moreover, the exposure to light of an MDF board incorporated into pieces of furniture is substantially longer and more intense than the conventional paper and causes a not inconsiderable yellowing.

It was therefore the object of the invention to provide light or white wood-base boards.

Accordingly, light to white wood-base boards which are produced from bleached wood fibers were found.

In addition, light to white wood-base boards which are beater dyed with a white pigment were found.

Furthermore, light to white wood-base boards which are beater dyed with a dispersion of a white pigment, which dispersion additionally comprises optical brighteners, were found.

Finally, light to white wood-base boards which are produced from bleached wood fibers and are beater dyed with a white pigment were found.

Not least, light to white wood-base boards which are produced from bleached wood fibers and are beater dyed with a dispersion of a white pigment, which dispersion comprises optical brighteners, were found.

The wood-base boards according to the invention are distinguished by their lightness or their white hue. Depending on the combination of measures taken, the desired whiteness can be easily established. Particularly white boards are obtained, for example, by bleaching the wood fibers and beater dyeing with a dispersion of a white pigment, which dispersion additionally comprises optical brighteners. Light boards having less whiteness are also obtainable only by bleaching the wood fibers or by beater dyeing with a white pigment.

The wood-base boards according to the invention may be MDF or HDF boards or particleboards MDF boards are particularly preferred.

MDF and HDF boards are usually produced in a continuous process. Washed, water-moist, finely chopped wood pieces (chips) are first preheated to about 80° C. and then softened in a digester under a pressure of from 2 to 5 bar and at a temperature of from 100 to 150° C. In the downstream refiner, the chips are then defibrated. The refiner consists of two metal disks which are provided with radial relief and which rotate close to one another in opposite directions. The fibers leave the refiner via the so-called blowline. Here, the glue is generally applied. Binders usually used are urea/formaldehyde resins, in some cases reinforced with melamine, or, for moisture-resistant boards, urea/melamine/formaldehyde resins. Isocyanates, too, are used as binders. The binders are generally applied together with the desired additives (for example, curing agent, paraffin dispersion, colorant) to the fibers. The fibers to which glue has been applied then pass through a drier in which they are dried to moisture contents of from 8 to 15% by weight. In a few isolated cases, the glue is also applied to the dried fibers only subsequently in special continuously operating mixers.

In particleboard production, the application of glue to the previously dried chips is effected in continuous mixers.

The fibers or chips to which glue has been applied are then poured to give mats, if appropriate precompacted in the cold and pressed in heated presses at temperatures of from 170 to 240° C. to give boards.

In the production of an embodiment of the light wood-base boards according to the invention, bleached wood fibers (below, no distinction is made between the terms “wood fibers” and “chips” and instead the term “wood fibers” is also intended to comprise “chips”) are used.

In the chemical bleaching of wood fibers, the color-imparting impurities of the wood are destroyed or rendered ineffective by oxidizing and/or reducing chemicals. For example, hydrogen peroxide, ozone, oxygen and salts of organic and inorganic per acids, such as peracetates, percarbonates and perborates, especially the alkali metal salts thereof, in particular sodium salts, are suitable for the oxidative bleach, the percarbonates and hydrogen peroxide being preferred. For example, reducing sulfur compounds, such as dithionites, disulfites, sulfites or sulfur dioxide, sulfinic acids and salts thereof, in particular the alkali metal salts and especially the sodium salts, and hydroxycarboxylic acids, such as citric acid and malic acid, are suitable for the reductive bleach. Preferred reducing agents are the disulfites and sulfites, in particular sodium hydrogen sulfite, and malic and citric acid.

For the wood-base boards according to the invention, the wood fibers which have been first oxidatively and then reductively bleached are particularly preferred.

The oxidative bleach is very particularly preferably carried out using percarbonates or hydrogen peroxide, and the reductive bleach using sulfites, malic acid or citric acid.

In the bleaching process, an expedient procedure is to treat aqueous 5 to 40% strength by weight wood fiber dispersions continuously in countercurrent towers at temperatures from 90 to 150° C. and pressures up to 3 bar with aqueous solutions or dispersions of the bleaches. Usually, the procedure is effected in the presence of complexing agents, such as EDTA, in order to avoid the degradation of the bleaches by transition metal ions

In the case of a preferred embodiment of the wood-base boards according to the invention, the bleaching of the fibers is carried out during the board production, particularly in the case of MDF/HDF boards. The bleaches may be added to the chips in the preheater or in the digester. Complexing agents are preferably also added.

In principle, all fibrous materials obtainable from plants may serve as base material for the wood-base boards according to the invention. Thus, for example, in addition to the usually used wood fibers, fibers which are obtainable from palms are suitable. Preferred base materials are light wood types, in particular spruce or pine, but darker wood types, such as beech, can also be used.

In a particularly preferred embodiment of the wood-base boards according to the invention, the use of bleached wood fibers is combined with beater dyeing with a white pigment. This makes it possible to improve the whiteness decisively. The beater dyeing with the white pigment can, however, also be carried out alone. In this case, light boards are likewise obtained.

According to the invention, the term “white pigment” comprises both inorganic pigments, such as titanium dioxide (rutile, C.I. Pigment White 6), calcium carbonate and mixed calcium/magnesium carbonates (e.g. dolomite), zinc oxide, zinc sulfide, lithopone and sodium aluminum silicates, as well as strongly light-scattering plastics emulsions and dispersions which impart whiteness. A particularly preferred white pigment is titanium dioxide. Of course, mixtures of white pigments can also be used.

The white pigments are preferably used in the form of aqueous dispersions, in which they are present in finely divided form, since they can in this form be introduced via the blowline, separately from or together with the glue, directly into the MDF/HDF production process.

These pigment dispersions may comprise further conventional assistants, in particular wetting agents and dispersants, antifoams and biocides, but also antisettling agents, water retention agents and rheology modifiers, and are preferably prepared by wet milling of all components, for example in a stirred ball mill.

Recommended concentrations of the white pigment in the prepared wood-base board are as a rule from 0.5 to 15%, preferably from 1 to 6%, based on dry fiber.

A further increase in the whiteness can be achieved by adding optical brighteners which, owing to their bluish fluorescence (complementary color), compensate graying and yellowing.

In principle, all blue-emitting fluorescent dyes, in particular commercially available products, e.g. Ultraphor® (BASF), Leucophor® (Clariant) or Tinopal® (Ciba), from the chemical classes of substances comprising stilbenes, distyrylbiphenyls, coumarins, and naphthalimides, and benzoxazole and benzmimidazole systems linked via double bonds, are suitable.

The optical brighteners may be introduced into the MDF/HDF production process in the form of aqueous dispersions or solutions, separately or together with the white pigments and the glue.

If an optical brightener is used, its concentration in the prepare d wood-base board is in general from 0.01 to 1%, preferably from 0.08 to 0.2%, based on dry fiber.

Those wood-base boards according to the invention in which all features (bleached wood fibers, white pigments and optical brighteners) are combined are very particularly preferred, since the individual contributions are enhanced by synergistic effects to give maximum overall whiteness.

With respect to the process, it is particularly advantageous to pre are the white pigments and the optical brighteners together in a single aqueous dispersion, which is added to the glue liquor before this is injected through the bromine into the MDF/HDF production process.

Any changes in the physical properties of the pressed wood-base board which result from the measures according to the invention can be controlled by the choice of the glue quality and amount of glue.

EXAMPLES

Production of MDF Boards According to the Invention

The lightness difference ΔL in comparison to an MDF boa rd produced analogously from unbleached wood fibers and without addition of white pigments and optical brighteners is determined as a standard by coloristic measurement according to CIELAB as a measure of the whiteness of the boards produced.

Example 1

-   a) In a 5 l vessel having an anchor stirrer and     thermostat-controlled heating, 70 g of wood pulp (spruce) and 1 g of     ethylenediaminetetraacetic acid (Trilon® BA BASF) in 3 l of water     were heated to 70° C. with stirring. After addition of 7 g of sodium     percarbonate, stirring was effected for 1 h at 70-750° C.     Thereafter, 7 g of sodium dithionite were added and finally stirring     was effected for a further 30 min a 70-75° C.     -   After cooling to room temperature, the wood slurry was separated         from the liquid components over a sieve of 1 mm mesh size and         washed briefly under running water and rolled out thoroughly.         The spread-out filtered material was then dried at 60° C. in a         through-circulation drying oven for 3 days.

b) The bleached wood fibers from step a) were thoroughly mixed in a paddle mixer and sprayed with the glue batch mentioned in table 1. TABLE 1 Glue batch Urea/melamine/formaldehyde resin, 100.0 parts by weight 69% strength by weight in water Paraffin dispersion, 4.1 parts by weight 60% strength by weight in water Water 49.2 parts by weight Solid resin content of the liquor 45% Solid resin/dry fibers 14% Liquor per 100 kg of dry fibers 31.1 kg

The fibers to which glue had been applied were then poured to give a mat, paecompacted in the cold and pressed at 190° C. to give a board.

Comparative Example C1

An MDF board was produced from the unbleached wood fibers analogously to the procedure described in example 1b).

Example 2

An MDF board was produced from unbleached wood fibers (spruce) and analogously to example 1b), the wood fibers being sprayed with the glue batch mentioned in table 2 and comprising the white pigment titanium dioxide. TABLE 2 Glue batch Urea/melamine/formaldehyde resin, 100.0 parts by weight 69% strength by weight in water Paraffin dispersion 4.1 parts by weight 60% strength by weight in water Titanium dioxide preparation, 39.4 parts by weight 70% strength by weight in water Water 9.8 parts by weight Solid resin content of the liquor 45% Solid resin/dry fibers 14% Titanium dioxide/dry fibers 5.6% Liquor per 100 kg of dry fibers 31.1 kg

Table 3 lists the whitenesses achieved (expressed in the lightness difference ΔL), based on comparative example C1 as standard. TABLE 3 Example ΔL 1 12 2 10 C1 —

Example 3

A 40% strength by weight aqueous sodium disulfite solution, corresponding to 7.5% of sodium hydrogen sulfite, based on dry fibers, was added to chips of spruce wood during the MDF production process in the digester of an MDF pilot plant. The MDF production process was continued as usual with a throughput of 21 kg/h, the chips were defibrated by the refiner, and the glue batch mentioned in table 4 was applied continuously through the blowline to the fibers obtained. TABLE 4 Glue batch Urea/melamine/formaldehyde resin, 100.0 parts by weight 66.5% strength by weight in water Paraffin dispersion 4.0 parts by weight 60% strength by weight in water Water 33.8 parts by weight Solid resin content of the liquor 48% Solid resin/dry fibers 14% Liquor per 100 kg of dry fibers 29.2 kg

The wood fibers to which glue had been applied were dried in the downstream continuous drier to a residual moisture content of about 9% by weight and then poured batchwise to give a mat, precompacted in the cold and pressed at 190° C. with a press time factor of 15 s/mm to give a 16 mm thick board.

Comparative Example C2

An MDF board was produced analogously to the procedure described in example 3, but without addition of sodium disulfite.

Table 5 shows the whiteness achieved (expressed as the lightness difference L), based on comparative example C2 as standard. TABLE 5 Example ΔL 3 4 C2 —

Example 4

Chips of spruce wood were sprayed in a mixer with a 40% strength by weight aqueous sodium disulfite solution, corresponding to 4.5% of sodium hydrogen sulfite, based on dry fibers. These chips were then fed into the digester of an MDF pilot plant. The MDF production process was continued as usual with a throughput of 21 kg/h, the chips were defibrated by the refiner, and the glue batch mentioned in table 6 and comprising the white pigment titanium dioxide was applied through the blowline continuously to the fibers obtained, TABLE 6 Glue batch Urea/melamine/formaldehyde resin, 100.0 parts by weight 66.5% strength by weight in water Paraffin dispersion 4.0 parts by weight 60% strength by weight in water Titanium dioxide preparation, 47.5 parts by weight 70% strength by weight in water Solid resin content of the liquor 44% Solid resin/dry fibers 14% Liquor per 100 kg of dry fibers 31.8 kg

The wood fibers to which glue had been applied were dried in the downstream continuous drier to a residual moisture content of about 9% by weight and then poured batchwise to give a mat, precompacted in the cold, and pressed at 190° C. with a press time factor of 15 s/mm to give 16 mm thick board.

Comparative Example C3

An MDF board was produced analogously to the procedure described in example 4 but without the addition of the white pigment (replacement of the titanium dioxide preparation by the same amount of water).

Table 7 shows the whiteness achieved (expressed as the lightness difference ΔL), based on comparative example C3 as standard. TABLE 7 Example ΔL 4 6 C3 —

Example 5

A 40% strength by weight aqueous sodium disulfite solution, corresponding to 6% of sodium hydrogen sulfite, based on dry fibers, was added to chips of beech wood during the MDF production process in the digester of an MDF pilot plant. The MDF production process was continued as usual with a throughput of 30 k/h, the chips were defibrated by the refiner, and the glue batch mentioned in table 8 was applied continuously through the blowline to the fibers obtained. TABLE 8 Glue batch Urea/melamine/formaldehyde resin, 100.0 parts by weight 66.5% strength by weight in water Paraffin dispersion 4.0 parts by weight 60% strength by weight in water Water 47.5 parts by weight Solid resin content of the liquor 44% Solid resin/dry fibers 14% Liquor per 100 kg of dry fibers 31.8 kg

The wood fibers to which glue had been applied were dried downstream in a continuous drier to a residual moisture content of about 9% by weight and then poured batchwise to give a mat, precompacted in the cold, and pressed at 190° C. with a press time factor of 15 s/mm to give a 16 mm thick board.

Example 6

An MDF board is produced using the glue batch mentioned in table 6, analogously to the procedure described in example 5 but with the use of the white pigment titanium dioxide.

Example 7

An MDF board was produced using the glue batch mentioned in table 9, analogously to the procedure described in example 5 but with the use of the white pigment titanium dioxide in combination with optical brighteners. TABLE 9 Glue batch Urea/melamine/formaldehyde resin, 100.0 parts by weight 66.5% strength by weight in water Paraffin dispersion 4.0 parts by weight 60% strength by weight in water Titanium dioxide preparation, 47.5 parts by weight 50% strength by weight in water, comprising 2% by weight of a commercial optical brightener Solid resin content of the liquor 44% Solid resin/dry fibers 14% Titanium dioxide/dry fibers 5% Optical brightener/dry fibers 0.2% Liquor per 100 kg of dry fibers 31.8 kg

Comparative Example C4

An MDF board was produced analogously to the procedure described in example 5 but without addition of sodium disulfite.

Table 10 shows the whitenesses achieved (expressed as the lightness difference ΔL), based on comparative example C4 as standard. TABLE 10 Example ΔL 5 3 6 6 7 7 C4 — 

1-5. (canceled)
 6. A light to white wood-base board which has been beater dyed with a white pigment.
 7. The wood-base board according to claim 6, to which optical brighteners have been added.
 8. The wood-base board according to claim 6, which is produced from bleached wood fibers.
 9. The wood-base board according to claim 6, which is produced from wood fibers which have been first oxidatively and then reductively bleached.
 10. The wood-base board according to claim 6, which may be an MDF board, HDF board or particleboard. 